1. Field of the Invention
It is possible to use both pressure-controlled and stroke-controlled injection systems to supply fuel to combustion chambers of autoignition internal combustion engines. In addition to unit fuel injectors, these fuel injection systems are also embodied in the form of unit pumps and accumulator (common rail) injection systems. Common rail injection systems, for example, advantageously permit the injection pressure to be adapted to the load and engine speed. It is generally necessary to achieve the highest injection pressure possible in order to achieve high specific loads and reduce engine emissions.
2. Description of the Prior Art
DE 101 23 910.6 relates to a fuel injection system that is used in an internal combustion engine. Fuel injectors supply fuel to the combustion chambers of the engine. A high-pressure source acts on the fuel injectors; the fuel injection system also includes a pressure booster that has a moving pressure boosting piston, which separates a chamber that can be connected to the high-pressure source from a high-pressure chamber connected to the fuel injector. The fuel pressure in the high-pressure chamber can be varied by filling a differential pressure chamber of the pressure booster with fuel or by emptying fuel from the differential pressure chamber of the pressure booster. The fuel injector has a moving closing piston for opening and closing injection openings. The closing piston protrudes into a closing pressure chamber so that fuel pressure can be exerted on the closing piston. This generates a force that acts on the closing piston in the closing direction. The closing pressure chamber and an additional chamber are comprised by a shared working chamber; all of the partial regions of the working chamber are connected to one another continuously to permit the exchange of fuel.
With this design, by triggering the pressure booster via the differential pressure chamber, it is possible to keep the triggering losses in the high-pressure fuel system significantly lower than a triggering by means of a working chamber that is connected to the high-pressure fuel source intermittently. In addition, the pressure in the high-pressure chamber is only relieved down to the pressure level of the common rail and not down to the leakage pressure level. On the one hand, this improves the hydraulic efficiency and on the other hand, it permits a more rapid pressure buildup to the peak pressure level so that the spaces of time between the injection phases can be shortened. According to the design known from DE 101 23 93913, the pressure booster and the injection nozzle are controlled by only a single valve, thus permitting the production of an inexpensive fuel injector that only takes up a small amount of space. This fuel injector permits a very high maximum injection pressure and features a variable hydraulic nozzle opening pressure so that even with small injection quantities, a high injection pressure can be achieved and the closing of the needle is significantly improved.
DE 102 29 417 A1 has disclosed a common rail injection system with a vario nozzle and a pressure boosting unit. A high-pressure fuel source supplies fuel to a fuel injector. A pressure booster is provided between an injection valve and the high-pressure fuel source. The pressure booster has a booster piston that separates a pressure chamber, which can be connected to the high-pressure fuel source, from a high-pressure chamber, which acts on a nozzle chamber of the fuel injector. The injection valve of the fuel injector has a nozzle needle that can open or close injection openings oriented toward a combustion chamber. The nozzle needle has a first nozzle needle part and an additional, second nozzle needle part, both of which are triggered in a pressure-dependent manner to open and close different injection cross sections in an injection nozzle. The two nozzle needle parts of the nozzle needle are guided one inside the other and have a surface that permits a hydraulic actuation. To this end, the first nozzle needle part has a pressure shoulder that can be actuated by means of the highly pressurized fuel flowing into a nozzle chamber. The second nozzle needle part has a pressure shoulder that is situated at the combustion chamber end of the second nozzle needle part.
The design known from DE 102 29 417 A1 permits the injection to be even better adapted to the requirements of the internal combustion engine. The opening pressure level of the inner needle part of the multipart injection valve member can be set to a constant, high level in a spring-assisted manner in order to prevent an opening in the partial load range of the internal combustion engine.
It has turned out that the setting of the opening pressure of the inner, second nozzle needle part of a multipart injection valve member is very tolerance-sensitive. An uncontrolled opening of the second, inner needle part of a multipart injection valve member leads to an abrupt jump in the injection quantity. Manufacture-conditional series tolerances with regard to the manufacturable tolerances therefore have a particularly negative effect with regard to the fuel supplied to the combustion chamber of an autoignition internal combustion engine.
With regard to the pressure fluctuations occurring in a common rail of the fuel injection system, therefore, the opening pressure of the inner, second needle part of a multipart injection valve member is very problematic. With regard to the opening pressure of the second, inner needle part of a multipart injection valve member, there is a critical pressure range during operation of the engine within which, due to the existing series tolerances of the fuel injectors and imprecisions in the pressure detection in the common rail, an indefinite opening of the second, inner needle part of the multipart injection valve member can occur, which can cause an indefinite quantity to be injected into the combustion chamber of the engine.